The present invention relates to an ebulliometric device for measuring with high precision a physical parameter of liquid substances, such as the temperature thereof, and to its application for determining the molar mass of substances.
The device is notably suited for determining molar masses by means of fine measurement of the boiling temperature difference between the boiling-point temperature of a pure solvent and that of a solution where a solute is dissolved in the same solvent.
The device according to the invention is suited for various applications in many fields, and notably for petroleum cut analysis in order to determine for example the proportion of each constituent distributed according to its chemical family and to the number of carbon atoms contained therein.
The principle of determination of the molar mass of a substance or of a mixture of substances by means of an ebulliometer is known in the art. It consists in measuring with precision, at a set pressure, the difference between the boiling-point temperature of a pure solvent and that of the same solvent in which the substance studied is dissolved, and in deducing its molar mass therefrom
An ebulliometer generally comprises an elongate vessel where the mixture to be measured is boiled at a controlled pressure, a temperature detector placed above the liquid phase, pumping means allowing to spray, in the vicinity of the temperature detector, a liquid-vapour mixture at equilibrium, taken at the base of the vessel, and recycling means including a condenser, for recycling the vapour phase to this base of the vessel.
Various ebulliometers are described for example by Olson J. D., xe2x80x9cMeasurement of Vapor-Liquid Equilibria by Ebulliometerxe2x80x9d in Fluid Phase Equilibria, 52 (1989) 209-218; Elsevier Science Publishers, or in patents FR-2,636,430 or AU-A-42,734/89.
The problem in this type of measurement is to obtain a stable temperature so as to reduce the possible temperature fluctuation margin, which requires close ebullition control so as to ensure that the temperature measured is the temperature at equilibrium of the mixture of liquid and gas phases.
The device according to the invention allows to measure with high precision a physical parameter of liquid substances (their boiling-point temperature or a characteristic depending thereon, linked with the composition thereof), comprising at least one ebulliometer including an elongate vessel into which a liquid substance is fed, means for boiling the liquid, a means for measuring the temperature, placed in the upper part of the vessel, a pumping means allowing to spray substance in the form of liquid and vapour towards the upper part of the vessel, means external to the vessel for condensing the vapour and allowing to recycle it to the base of the vessel.
It is characterized in that it comprises a first tube into which is lowered the means for measuring the temperature in the upper part of the enclosure, a second tube delimiting a containment space around the first tube, this space communicating with the pumping means, a means associated with the first tube for increasing the liquid/vapour contact time in the containment space, means for homogenizing the boiling temperature and at least one thermostat-controlled enclosure.
According to an embodiment, the device comprises two identical ebulliometers into which two liquids are respectively fed, one being a pure solvent and the other a solution of a solute in this solvent, a means for measuring the difference between the temperatures prevailing respectively in the containment spaces delimited by the second tubes of the two ebulliometers, and a means for determining the molar mass of the solute from the measured temperature difference.
According to another embodiment, the means for homogenizing the boiling temperature in each ebulliometer include circuits for delivering gas into the liquid.
According to another embodiment, each ebulliometer comprises a pointed element intended to prevent formation of liquid droplets in the vicinity of the measuring means.
According to another embodiment, the means for condensing the vapour in each ebulliometer comprise a circuit external to the vessel, suited to recycle the condensed liquid by thermosiphoning, this circuit communicating by means of a refrigerating column with a pressure and discharge system.
According to another embodiment, the pressure and discharge system comprises for example a moisture absorption column.
The means for boiling each liquid substance comprise for example a heat plug in each ebulliometer, these plugs being connected to a common power supply.
According to an embodiment, the first thermostat-controlled enclosure is a solvent vapour oven provided with an external circuit for vapour condensation and recycling by thermosiphoning, which communicates with a refrigerating column and with said pressure and discharge system, the device also comprising a second enclosure including the first enclosure with its external circuit, and heating and temperature control means in this second enclosure.
The stability obtained by the device is of the order of one ten thousandth of a degree (0.0001), which allows to precisely determine molar masses of solute around 10,000 g/mol with solute concentrations below 1 g/1000 g solvent.
As shown in the description hereafter, various factors contribute to the stability of the temperature obtained in each ebulliometer and in the two ebulliometers with a comparative assembly.